1. Field of the Invention
The present invention relates to a method and apparatus for the electrostatic application of powder material onto the surfaces of solid dosage forms, and more particularly, but not exclusively, pharmaceutical solid dosage forms.
2. Description of Related Art
Including information disclosed under 37 CFR 1.97 and 1.98
A “solid dosage form” can be formed from any solid material that can be apportioned into individual units; it may be, but is not necessarily, an oral dosage form. Examples of pharmaceutical solid dosage forms include pharmaceutical tablets, pharmaceutical pessaries, pharmaceutical bougies and pharmaceutical suppositories. The term “pharmaceutical tablet” should be interpreted as covering all pharmaceutical products which are to be taken orally, including pressed tablets, pellets, capsules and spherules. Examples of non-pharmaceutical solid dosage forms include items of confectionery and washing detergent tablets.
The electrostatic application of powder material to solid dosage forms is known. In one technique, described in WO 96/35516 powder material is applied onto pharmaceutical tablets while the tablets are moving on a drum past a source of the powder material. The tablets are supported in cupped receptacles on a first drum and all the exposed areas of the tablets are coated as they pass the source of powder material. Subsequently the tablets are transferred onto a second drum where they are supported again in cupped receptacles but in the opposite orientation to that on the first drum so that areas of the tablets not exposed on the first drum are now exposed and vice versa. In that way the whole of each tablet is coated following its passage around both drums.
When using the apparatus of WO 96/35516 we have found that some powder is applied to the surface of the drum as well as to the tablet. That is wasteful of powder and also makes cleaning of the apparatus time consuming, especially if the powder being applied by the apparatus is to be charged. The coating of the sides of the tablets using the apparatus of WO 96/35516 can also be somewhat arbitrary: portions of the sides are liable to be exposed during coating on each of the drums and may therefore acquire more powder than the ends of the tablets; on the other hand, the amount of powder reaching the sides of the tablets may be limited so that even after both coating stages relatively little powder is applied to the sides of the tablet. Also, it is sometimes desired to coat only one half of the tablet (one end and part of the side wall) and in that case it is desirable to have a well defined edge to the coating. It is difficult to provide such a coating with a well defined edge using the apparatus of WO 96/35516.